The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenitic duplex stainless steel can undergo a variety of decomposition processes when aged in the temperature range 650-550 degreesC. One of these processes is the formation of austenite in the shape of a spearhead. Unlike the one found at high temperature, this austenite is characterized by a habit plane, which is similar to the midrib of the martensite. This feature suggests that a diffusionless process as referred to in the literature controls its formation. In the present investigation, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) results indicate that the proposed precipitation mechanism must be reviewed. Based on the crystallographic results in conjunction with the clear-cut difference in chemical composition between the matrix and the austenitic second phase, the equilibrium shape is explained and the ambiguity of precipitation mechanism has been brought to light. It has been suggested that the latter one goes through the following steps: (i) enrichment of the (110)-ferritic planes with gamma -forming elements by diffusion, (ii) double shear to transform (110)-ferritic planes into (111)-austenitic planes, i.e., to change from BCC (delta) to FCC (gamma), (iii) twinning of the FCC gamma structure to reduce local strains and formation of gamma bi-crystals and (iv) growth of the twinned gamma towards the ferritic matrix by volume diffusion.